The invention relates to a system and a procedure for locating a mobile telephone within a wide geographic surface covered by a mobile telephone service network.
Mobile telephone service is steadily expanding and it is estimated that the number of mobile subscribers will increase on a world-wide level by an order of magnitude within the next ten years.
With the expansion of the service, the problem of locating a mobile subscriber moving from one area to another (roamer) within the range of a wide geographic surface has become of primary importance.
Different mobile telephone systems are presently available. An example of such a system is the cellular one, described in "The Bell System Technical Journal", January 1979 issue, vol. 58, no 1, page 1-269, to which the reader is referred for further details. This system allows the location of a mobile unit or subscriber within the range of a single area divided into cells, but does not tackle the problem of locating any such vehicle in a very large surface which includes several areas.
A known solution to the problem is based on the mobile unit sending its identity signal (and consequently notifying its presence) to the different mobile telephone exchanges (CM) as it crosses the respective areas. Each CM exchange will then inform all the other CM exchanges, provided with a memory, of the presence of the mobile unit at that particular moment. When the mobile unit crosses over into another area, the associated CM exchange, upon receiving an identity signal from the telephone unit, will transmit it to all the other CM exchanges together with its own identity signal, for the purpose of updating the mobile unit's position.
It is obvious that this system can only operate with a limited number of subscribers since it involves the processing by the CM exchanges of a considerable amount of data. This data may turn out to be completely useless should the mobile subscriber receive no calls.
Other solutions are based on the mobile units identity and position messages sent by the exchanges whose areas are crossed by the specific unit to a single center. This center may be the center to which the unit is assigned.
Any CM exchange whic contacts this center may receive the information required for making the connection. In this case too a certain amount of data must be exchanged. The data exchanged is less than in the previously described case, but is just as useless if the mobile subscriber receives no call.
Systems using this solution are the Nordic Mobile Telephone, a Japanese system iillustrated in the document ISS'84 Florence, section 32-B, paper 4, and others.